The disclosures of the following priority applications are herein incorporated by reference:
Japanese Patent Application No. 2001-285414 filed Sep. 19, 2001; and
Japanese Patent Application No. 2002-152632 filed May 27, 2002.
1. Field of the Invention
The present invention relates to a zoom lens system having a vibration reduction correction mechanism suitable for single lens reflex cameras and electronic still cameras, in particular, to a large aperture internal focusing telephoto zoom lens system having the focal length in the telephoto end state of 180 mm or more, the zoom ratio of about 2.7 or more, and the f-number of about three or less.
2. Related Background Art
An optical system whose imaging position is corrected by shifting a portion of lenses or lens groups in the optical system in the direction perpendicular to the optical axis has been used in this kind of optical system as disclosed in Japanese Patent Application Laid-Open Nos. 2-234115 and 9-325269.
However, in the optical system disclosed in Japanese Patent Application Laid-Open No. 2-234115, it is inevitable for the driving actuator to become large in order to control the master lens group of the zoom lens precisely in the direction perpendicular to the optical axis, so that it is not practical. In addition, the optical system disclosed in Japanese Patent Application Laid-Open No. 2-234115 does not mention imaging performance. Moreover, in the optical system disclosed in Japanese Patent Application Laid-Open No. 9-325269, although the driving actuator is able to be small and correction of aberration is preferable since vibration reduction correction is carried out by a portion of the master lens, the f-number is merely four, so that it is not suitable for sports photography.
In this kind of a large aperture telephoto zoom lens using a lens group for focusing suitable for single lens reflex cameras and electronic still cameras, the lens group used for focusing by moving along the optical axis has had a rather large diameter.
However, the above-described large aperture telephoto zoom lens has had disadvantages that because of large diameter of the focusing lens group, the weight of the focusing group is large and heavy load acts to the motor while carrying out auto focusing (AF), so that the large consumption of battery shortens the life time of the battery. Moreover, since the moving distance of the focusing lens group is large for focusing, it takes long time for carrying out auto focusing, so that it is not suitable for quick shooting.
In order to solve the problem, a focusing method has been proposed in Japanese Patent Application Laid-Open No. 6-51202, in which a first lens group fixed upon zooming is divided into two portions of a front lens group having positive refractive power and a rear lens group having positive refractive power, and the rear lens group is used as a focusing lens group being moved along the optical axis. In examples disclosed in Japanese Patent Application Laid-Open No. 6-51202, although the weight of the focusing lens group can successfully be light by constructing the focusing lens group with a single positive lens, the closest focusing distance is very far about 8.3-8.4 m in the telephoto end state, so that it is not acceptable for general shooting. If the focusing lens group is moved up to the shooting distance of 1.5 m, which is the same distance as the present application, then the moving distance of the focusing lens group becomes very large about 16.6-18.7 mm, so that it cannot carry out quick auto focusing. Moreover, since only a single positive lens is used for the focusing lens group, spherical aberration of the focusing lens group is not corrected, so that variation in spherical aberration upon focusing in the telephoto end state becomes too large to be accepted as a general shooting lens.
In order to solve the problem described above, in examples disclosed in Japanese Patent Application Laid-Open Nos. 7-294816 and 2001-356381, a lens portion equivalent to the rear lens group disclosed in Japanese Patent Application Laid-Open No. 6-51202 is moved to the image upon zooming and is moved independently along the optical axis upon focusing. Accordingly, the effective diameter of the focusing lens group can be small, so the focusing lens group can be made light. Moreover, in examples disclosed in Japanese Patent Application Laid-Open No. 7-294816, the moving distance of the focusing lens group can successfully be made short about 8.3-13.4 mm. Furthermore, spherical aberration of the focusing lens group can be corrected by constructing the focusing lens group with two-group two-element of a negative meniscus lens and a positive meniscus lens, so that variation in spherical aberration upon focusing (in particular, in the telephoto end state) is reduced.
On the other hand, since the focusing lens group is moved upon zooming, the moving amount of the focusing lens group upon focusing varied in accordance with the focal length. This means that the lens is no more a zoom lens but a variable focal length lens. Therefore, in order to be able to handle the lens system just like a zoom lens, a connecting member for connecting a focusing barrel with a zooming cam barrel upon zooming had to be added. Accordingly, the connecting member for connecting with the zooming lens groups is provided in addition to the lens barrel for the focusing lens group, the load for auto focusing becomes heavy, so that it is not preferable for driving AF motor. Moreover, in terms of lens assembling, when the focusing lens group, which is the front lens group of a telephoto type lens system, is decentered, the image plane tends to tilt severely. However, since the play of the focusing lens group disclosed in Japanese Patent Application Laid-Open No. 7-294816 is the sum of the play of the moving part for focusing and that of the connecting member with the zooming lens groups, the play produces a large decentered amount, so that it is extremely difficult for the actual product to maintain the flatness of the image plane.
Furthermore, in examples disclosed in Japanese Patent Application Laid-Open Nos. 2000-19398 and 2001-356381, the above-described problems are solved such that a first lens group of a four-group afocal zoom lens is divided into a front group and a rear group, and the distance between the front group and the rear group is widened. However, in order to accomplish the specification of the focal length in the wide-angle end state of about 80 mm, and the zoom ratio of about 2.4, the total lens length tends to become long because the widened distance becomes dead space.
In spite of being small in the effective diameter of the focusing lens group, the thickness along the optical axis of the positive lens in the focusing lens group is relatively thick. If the zoom ratio is made higher without changing lens type, the effective diameter of the focusing lens group is obliged to be larger. Accordingly, the thickness along the optical axis of the positive lens in the focusing lens group is obliged to be thick, so that the increase of the weight in accordance with the increase of the volume is disadvantageous to the focusing response.
The present invention was made in view of the aforementioned problems and has an object to provide a zoom lens system having a relatively small thickness along the optical axis of a positive lens in the focusing lens group, and a vibration reduction correction mechanism with keeping superior optical performance and, in particular, a large aperture internal focusing telephoto zoom lens having the focal length in the wide-angle end state of 72 mm or less, the focal length in the telephoto end state of 180 mm or more, the zoom ratio of 2.7 or more, and the f-number of 3 or less.
According to one aspect of the present invention, an afocal zoom lens system includes, in order from an object, a first lens group having positive refractive power, a second lens group having negative refractive power, a third lens group having positive refractive power, and a fourth lens group having positive refractive power. Zooming is carried out by moving the second lens group and the third lens group along the optical axis. The fourth lens group is composed of, in order from the object, a front group having positive refractive power, a middle group having negative refractive power, and a rear group having positive refractive power. An imaging position is varied by shifting the middle group in the direction perpendicular to the optical axis. The front group includes a positive lens and a negative lens. The middle group includes a positive lens and two negative lenses. The rear group includes two positive lenses and a negative lens. The following conditional expression is satisfied;
0.70 less than |(F4xc3x97F4M)/(F4Fxc3x97F4R)| less than 1.20 
where F4 denotes the focal length of the fourth lens group, F4F denotes the focal length of the front group, F4M denotes the focal length of the middle group, and F4R denotes the focal length of the rear group.
In one preferred embodiment of the present invention, the following conditional expression is satisfied;
0.008 less than F4/(F4Fxc3x97F4Rxc3x97Nd) less than 0.015 
where Nd denotes the average refractive index at d-line of the lens elements consisting of the middle group in the fourth lens group.
In one preferred embodiment of the present invention, the following conditional expression is satisfied;
0.40 less than |("PHgr"Fxc3x97F4R)/(F4xc3x97"PHgr"M)| less than 0.80 
where "PHgr"F denotes the maximum effective diameter of the front group, and "PHgr"M denotes the maximum effective diameter of the middle group.
In one preferred embodiment of the present invention, the following conditional expression is satisfied;
0.70 less than |(FTxc3x97F23Txc3x97F4M)/(F1xc3x97F4Fxc3x97F4R)| less than 1.20 
where FT denotes the focal length of the zoom lens system in the telephoto end state, F1 denotes the focal length of the first lens group, and F23T denotes the composite focal length of the second lens group and the third lens group in the telephoto end state.
In one preferred embodiment of the present invention, the front group is composed of two positive lenses and a negative lens, and the rear group is composed of two positive lenses and a negative lens.
In one preferred embodiment of the present invention, the first lens group having positive refractive power is composed of a front lens group fixed along the optical axis and a rear lens group movable along the optical axis. Focusing is carried out by moving the rear lens group along the optical axis.
According to another aspect of the present invention, a telephoto zoom lens system includes, in order from an object, a first lens group having positive refractive power, a second lens group having negative refractive power, a third lens group having positive refractive power, and a fourth lens group having positive refractive power. Zooming is carried out by moving the second lens group and the third lens group along the optical axis. The first lens group is composed of, in order from the object, a front lens group having positive refractive power, and a rear lens group having stronger positive refractive power than that of the front lens group. Focusing is carried out by moving the rear lens group along the optical axis. The front lens group is composed of, in order from the object, a negative meniscus lens having a convex surface facing to the object, a positive lens having a convex surface facing to the object, and a positive lens having a convex surface facing to the object. The rear lens group is composed of, in order from the object, a negative meniscus lens having a convex surface facing to the object, and a positive lens. The following conditional expressions are satisfied;
0.5 less than (F1xc3x97F4)/(|F23W|xc3x97FW) less than 1.2 
1.0xc3x9710xe2x88x923 less than xcexd15/(F1xc3x97F1Rxc3x97N15) less than 2.6xc3x9710xe2x88x923 
where FW denotes the focal length of the zoom lens system in the wide-angle end state, F1 denotes the focal length of the first lens group, F1R denotes the focal length of the rear lens group of the first lens group, N15 denotes the refractive index of the positive lens in the rear lens group at d-line, xcexd15 denotes Abbe number of the positive lens in the rear lens group, F23W denotes the composite focal length of the second lens group and the third lens group in the wide-angle end states, F4 denotes the focal length of the fourth lens group.
In one preferred embodiment of the present invention, the following conditional expression is satisfied;
0.10 less than |F23T|/|F23W| less than 0.45 
where F23T denotes the composite focal length of the second lens group and the third lens group in the telephoto end state.
In one preferred embodiment of the present invention, the following conditional expression is satisfied;
0.015 less than FW/(F1xc3x97|F2|) less than 0.028 
where F2 denotes the focal length of the second lens group.
Other features and advantages according to the invention will be readily understood from the detailed description of the preferred embodiments in conjunction with the accompanying drawings.